This work relates to nuclear magnetic resonance (NMR) apparatus and particularly to the RF coupling to nuclei of the sample under study. This is the function of a module widely termed the NMR probe, which controls the distribution of the RF field within a sensitive region. A sample within the sensitive region is closely coupled to the RF radiation generated in the sensitive region for spin excitation and subsequently emitted with the de-excitation of sample nuclei. The heart of the probe is the RF coil and the salient property of such coil in typical use is the degree of homogeneity of the RF field achieved over the interior of the sensitive volume defined by the coil. It is also desired to constrain the RF field distribution for the coil to a limited region within the sensitive volume because the spatial variation of the polarizing magnetic field is not eliminated outside of the sensitive volume.
Limiting the spatial distribution of the RF field of the NMR coil is the subject of a number of prior art works. Of particular interest for this purpose are U.S. Pat. Nos. 6,008,650 and 5,192,911, both commonly assigned herewith. In general these works describe shielding implemented to protect the sensitive volume of the resonance apparatus from RF influence external thereto, or, to reduce the RF field of the coil outside of the sensitive volume. For example, it is desired to shield that portion of the sample extending beyond the sensitive volume from irradiation arising from the coil or the coil leads. This does not protect that same portion of the sample (disposed in a possibly slightly different polarizing field) from irradiation due to the RF field distributed predominantly, but not completely in that sensitive volume. In general these shielding arrangements include a conducting member, typically of cylindrical form, coaxial with the sample axis and axially displaced from the central region of the RF coil. Such shielding predominantly attenuates the radial components of the RF field in the axial region beyond the shield structure.
Another approach to the problem of undesired excitation of sample outside the sensitive volume is based upon physically limiting the sample volume to coincide with the axial extent of the probe coil. In order to avoid axial discontinuity in magnetic susceptibility, the prior art utilized plugs, susceptibility matched to the sample, and inserted into the sample vessel to confine the sample to the desired region, coincident with the probe coil. This is disclosed by U.S. Pat. No. 4,549,136 to Zens, commonly assigned herewith.
RF cavity resonators are known for NMR apparatus and such resonators effectively contain the internal RF field and therefore shield the sample volume from RF influence external to the cavity. A representative example of such NMR cavity resonator is disclosed in U.S. Pat. No. 4,607,224, commonly assigned herewith. Although the cavity structure provides both axial and radial constraint to the field distribution, the ability is lost to impose an independent RF field on the sample from outside of such resonator. This is a necessary tool for spin decoupling and for a number of multiple resonance techniques.